The objective of this proposed study is the refinement and testing of our new approach to retinal blood flow measurement, the fluorescent vesicle system, which uses a scanning laser ophthalmoscope, and a digital image analysis system capable of recording and digitizing video angiograms to view the movement of vesicles containing a subquenched concentration of dye through the retinal macrocirculation, the macular and optic nerve head microcirculation, and the choroidal vessels. To this end, we propose the following specific aims: 1) to determine in vitro the most efficient concentrations of encapsulated dyes in order to maximize the fluorescence of the vesicles, using a model of retinal vessels to evaluate various combinations of vesicle size, dye and dye concentration; 2) to determine how vesicle size, phospholipid composition, charge, and structure of the liposomes affect the survival time and clearance of the fluorescent vesicles from the circulation by testing in vivo first in the rabbit eye and then in the monkey eye; 3) to map blood flow in the normal retina in a nonhuman primate using the fluorescent vesicle system; tracing the movement of well-defined fluorescent vesicles on overlays constructed from successive video frames will allow calculation of the velocity of blood flow in large and small vessels of the retina, macula, optic nerve head, and choroid; 4) to investigate hemodynamic responses to a pharmacological agent that alters blood viscosity (dextran) and a drug that causes vasodilation (anisodamine) in the nonhuman primate retina; changes produced by these agents can be monitored simultaneously and objectively in the entire retinal macrocirculation and the macular microcirculation; and 5) to map choroidal blood flow in a normal nonhuman primate eye using the fluorescent vesicle system; two kinds of fluorescent vesicles separately encapsulated with two dyes will be used. The fluorescent vesicle system overcomes the limitations of scanning laser ophthalmoscopy in measuring blood flow. It allows objective measurement of blood flow velocity in any size vessel. Diabetic retinopathy is a blinding eye disease involving retinal blood flow. Diagnosis of complications of diabetes before retinopathy develops, as well as evaluating the effectiveness of treatments such as pharmacological agents and panretinal photocoagulation, may be improved by using the fluorescent vesicle system. Management of glaucoma may be improved with optic nerve head capillary blood flow data provided with this approach. Impending central retinal vein occlusion can also be diagnosed by determining retinal blood flow, allowing treatment to begin earlier.